The hydrofluorination of 1,1,1-trichloroethane to form 1,1-dichloro-1-fluoroethane and 1-chloro-1,1-difluoroethane generates 1,1,1,3,3-pentafluorobutane (hereinafter "pentafluorobutane") as a by-product. While pentafluorobutane is generated only in small amounts, typically about 0.5 wt% based upon the amount of 1,1-dichloro-1-fluoroethane product, pentafluorobutane and 1,1-dichloro-1-fluoroethane have very similar boiling points, respectively 40.degree. C. and 32.degree. C. They also form an azeotrope, having a composition of about 19 mol% pentafluorobutane and 81 mol% 1,1-dichloro-l-fluoroethane. Thus, separation of pentafluorobutane from 1,1-dichloro-1-fluoroethane by simple distillation is not feasible. A significant loss of 1,1-dichloro-1-fluoroethane would result from distilling the pentafluorobutane/1,1-dichloro-1-fluoroethane azeotrope since the composition of the azeotrope is about 19 mol% pentafluorobutane to 81 mol% 1,1-dichloro-1-fluoroethane.